The present invention relates to an independent ignition type ignition coil for an internal combustion engine which is mounted for each of respective ignition plugs for the internal combustion engine and is directly coupled each of the respective ignition plugs.
These days an independent ignition type ignition coil device for an internal engine has been developed which is used after being mounted in each of plug holes in the engine and being directly coupled to each of the respective ignition plugs. The ignition coil device of this sort unnecessitates a distributor, as a result, the decreasing of supply energy to an ignition coil through the distributor, high voltage codes therefor and the like is eliminated, moreover, since the ignition coil can be designed without taking into account of the ignition energy decreasing, it is evaluated that the voltage for the ignition coil can be reduced and the size reduction of the ignition coil is achieved as well as because of the elimination of the distributor the spacing for mounting a variety of parts in an engine room is rationalized.
The ignition coil of such independent ignition type is called as an in-plug mounting type, since at least a part of the coil portion is introduced into a plug hole and is mounted or fitted there, further, the coil portion is commonly called as a pencil coil, since the coil portion is shaped into a long and slender pencil so as to permit insertion the same into the plug hole, and inside a long and slender cylindrical casing a center core (which is an iron core made magnetic flux passage and is formed by laminating many silicon steel sheets), a primary coil and secondary coil are disposed. Through conduction and interruption control of a current flowing through the primary coil a high voltage necessary for ignition is generated in the secondary coil, therefore, these coils are usually wound around respective bobbins and are disposed concentrically around the center core. The insulation property for the coils is guaranteed such as by filling (hardening after filling) an insulation use resin and by sealing an insulation oil into the coil casing accommodating the primary and secondary coils. For example, JP-A-8-255719, JP-A-9-7860, JP-A-9-17662, JP-A-8-93616, JP-A-8-97057, JP-A-8-144916, and JP-A-8-203757 disclose prior art of the present invention.
There are two types of pencil coils, in one the primary coil is disposed inside and the secondary coil is disposed outside, and in the other the secondary coil is disposed inside and the primary coil is disposed outside. Among these two, the entire wire length of the secondary coil in the latter type (inner secondary coil structure) is short in comparison with that in the former type (outer secondary coil type) and the electrostatic stray capacity at the secondary side thereof is also small, therefore, the inner secondary coil structure is understood advantageous with regard to its output characteristic.
Namely, the secondary output voltage and its building up characteristic are affected by the electrostatic stray capacity and when the electrostatic stray capacity increases, the output voltage reduces and the building up thereof is caused to delay. Accordingly, it is considered that the inner secondary coil structure which has a small electrostatic stray capacity is suitable for reducing the size thereof and for raising the output voltage.
Among these sorts of the ignition coil devices of the independent ignition type, a type which uses the insulation use resin (for example, epoxy resin) filled between the constituting members (between such as a center core, bobbins and coils and between such as layers of the coils) in the coil casing eliminates a measure for sealing which is necessitated such as in the insulation oil sealing type, further, the constituting members thereof such as the center core, the bobbins and the coils are by themselves secured only by burying the same into the insulation use resin, therefore the measure for securing the constituting members is simplified in comparison with the insulation oil sealing type and thus it is evaluated that a simplification of the total device and handling easiness thereof are achieved.
Since as the insulation use resin between the constituting members of the ignition coil device an epoxy resin is injected and hardened (filled), and since the hardening temperature of such epoxy resin is usually more than 100xc2x0 C., under a low temperature less than the hardening temperature such as the insulation use resin the bobbin material are subjected to a thermal stress based on linear expansion coefficient differences between the constituting members (in that linear thermal expansion differences between such as the bobbins, coils, center core and the insulation use resin), therefore, it is necessary to take some measures for preventing possible crackings and interface peeling-offs between the members due to the thermal stress.
For example, in case of the inner secondary coil structure type;
(1) First of all, it is an important point how to reduce a thermal stress between the center core and the secondary coil bobbin of which linear expansion coefficient difference is large. For this purpose the following measures, for example, are taken, in that as the insulation use resin to be filled between the center core and the secondary coil bobbin such as a soft epoxy resin having a soft property at least above a normal temperature (a flexible epoxy resin; elastomer) is used in place of a hard epoxy resin so as to absorb a thermal impact, and in that after inserting a center core covered in advance by an insulation member having an elasticity into the secondary coil bobbin, the entire structure is sealed by a hard epoxy resin to ensure insulation property thereof.
(2) A primary factor of causing cracks in the bobbin material is understood to be an internal stress (thermal stress) of the bobbins due to linear expansion coefficient differences between the center core, the primary coil, the secondary coil and the bobbins (resin), in particular in case of the inner secondary coil structure type, it was clarified by the present inventors through a heat cycle testing (a heat cycle test of 130xc2x0 C.xcx9cxe2x88x9240xc2x0 C.) that the cracking (of which cracking is so called longitudinal cracking developing into the axial direction of the bobbin) is most likely caused in the secondary coil bobbin among both bobbin materials (the heat cycle test of 130xc2x0 C.xcx9cxe2x88x9240xc2x0 C. assumes a severe engine use environment condition in cold districts).
This crack generation mechanism in the secondary coil bobbin is caused, because the linear expansion coefficient of the bobbin material is large in comparison with those of the center core and the coil material. Namely, when the ignition coils are subjected to thermal contraction due to temperature drop after stopping of the engine operation, a thermal contraction of the secondary coil bobbin, in particular the degree of the thermal contraction in its circumferential direction is much larger than those of the center core and the coil materials (the primary coil and the secondary coil). Accordingly, when the secondary coil bobbin tends to undergo a thermal contraction, at the inside thereof the center core is subjected to the thermal contraction force (when the resin interposed between the secondary coil bobbin and the center core is an elastomer such as a soft epoxy resin, the center core is subjected to the thermal contraction force of the secondary coil bobbin at a temperature less than the glass transition temperature thereof), as a result, the secondary coil bobbin is applied relatively of a force from the side of the center core in relation to the center core and is subjected to an expansion force in the circumferential direction. Further, when the secondary coil bobbin tends to undergo a thermal contraction, the primary coil and the secondary coil of which linear expansion coefficients are smaller than that of the secondary coil bobbin act so as to suppress the thermal contraction of the secondary coil bobbin via the insulation use resin (in other words, a tension force in the circumferential direction is provided to the secondary coil bobbin). Due to these multiple actions a large internal stress (thermal stress) a is generated in the secondary coil bobbin and causes longitudinal direction crackings in the secondary coil bobbin.
Such longitudinal direction cracking in the secondary coil bobbin causes an electric field concentration between the center core and the secondary coil and finally leads to an insulation breakdown.
An object of the present invention is to improve an independent ignition type ignition coil which is mounted in a plug hole and is subjected to a severe temperature environment, in that to prevent the above mentioned crackings in the secondary coil bobbin, to hold a soundness of an electric insulation performance thereof, and to achieve a high quality and high reliability of the concerned type ignition coil device.
The present invention primarily proposes the following task resolving measures for achieving the above object.
(1) Namely, an independent ignition type ignition coil for an internal combustion engine according to a first aspect of the present invention which is used after being inserted into a plug hole in the internal combustion engine and being directly coupled to a corresponding ignition plug, and which includes a center core, a secondary coil wound around a secondary coil bobbin and a primary coil wound around a primary coil bobbin arranged concentrically in a coil casing in this order from the inside of the coil casing and an insulation use resin filled between the constituting members in the coil casing, is characterized in that between the primary coil bobbin and the primary coil and/or between layers of the primary coil a gap portion which reduces a stress component caused inside the secondary coil bobbin due to thermal contraction difference of the primary coil and the secondary coil bobbin among thermal stress caused inside the secondary coil bobbin is coexisted with the insulation use resin.
The gap is obtained by forming a peeling off portion at least one, between the primary coil bobbin and the insulation use resin (for example, an epoxy resin) filled between the primary coil bobbin and the primary coil, between the insulation use resin filled between the primary coil bobbin and the primary coil and the primary coil and between the primary coil and the insulation use resin filled between the layers of the primary coil.
More specifically, the present invention proposes such as to apply on the primary coil a cover film or a cover coating which facilitates peeling off of the primary coil from the insulation use resin filled around the primary coil, to apply on a side of bobbin surfaces (the outside surface of the bobbin) of the primary coil bobbin on which the primary coil is wound a cover film or a cover coating which facilitates peeling off of the insulation use resin contacting the bobbin surface from the bobbin surface, and in place of these cover film and cover coating to adhere an insulation sheet having a weak adhesiveness to an epoxy resin on the primary coil. As a material for the cover film or the cover coating material having a slipping property, such as nylon, polyethylene and teflon and an overcoating containing in an insulation material a material having a small adhesiveness to an epoxy resin are exemplified.
When temperature lowers after hardening the epoxy resin a tension force acts at the interfaces between the epoxy resin and the primary coil or the primary coil bobbin due to the linear expansion coefficient difference between the epoxy resin and the primary coil material copper, and a peeling off will be caused at a portion having a weak adhesiveness with the epoxy resin.
The principle of the present invention is as follows, in that when the ignition coil tends to undergo a thermal contraction due to temperature drop after stopping of the engine operation, the secondary coil bobbin is subjected relatively to an expansion force in the circumferential direction from the side of the center core due to the thermal contraction difference (the linear expansion coefficient difference), further, the secondary coil bobbin is subjected relatively to a tension force in the circumferential direction from the side of the primary coil and the secondary coil via the insulation use resin and with these multiple actions a large internal stress "sgr" is generated in the secondary coil bobbin. However, according to the present invention, a gap (for example, the above peeling off portion) is interposed between the primary coil bobbin and the primary coil and/or between the layers of the primary coil, thereby, a transmission passage of the tension force in the circumferential direction acting from the primary coil to the secondary coil bobbin can be interrupted.
Accordingly, among the stress "sgr"1 caused in the secondary coil bobbin a stress component "sgr"1 caused in the secondary coil bobbin due to the thermal contraction difference between the primary coil and the secondary coil bobbin is reduced, thereby, the total internal stress "sgr" can be greatly reduced (relaxed). According to CAE (Computer Aided Engineering) analysis examples performed by the present inventors, through the reduction of the above mentioned stress component "sgr"1 it is determined that at least 20% of the total internal stress can be reduced. Further, such reduction value in the internal stress was confirmed by making use of an ignition coil which is used after being inserted into a plug hole in an internal combustion engine and being directly coupled to a corresponding ignition plug and of which portion being inserted into the plug hole has an outer diameter of 18 mmxcx9c27 mm (in a long and slender cylindrical type ignition coil of this sized, usually the thickness of the primary coil bobbin is 0.5 mmxcx9c1.2 mm, the thickness of the secondary coil bobbin is 0.7 mmxcx9c1.6 mm and the length of the bobbins is 0.5 mmxcx9c150 mm).
Further, it was confirmed through experimental results that even if the above mentioned gap (for example the peeling off portion) is provided between the primary coil bobbin and the primary coil and/or between the layers of the primary coil, no electric field concentration between the primary coil is caused because of a low potential (substantially at the ground potential) of the primary coil, in addition if the secondary coil, the insulation use resin and the primary coil bobbin are closely bonded without gaps, the insulation between the primary coil and the secondary coil can be sufficiently ensured, moreover, a possible electric field concentration due to the line voltage of the secondary coil can be sufficiently prevented, thereby a possible generation of insulation breakdown can be prevented. (2) Further, in addition to the above explained first aspect of the present invention, for example, when a denaturated PPE (denaturated polyphenylene-ether) is used for the secondary coil bobbin, and if in view of material property improvement of the secondary coil bobbin, more than 20 weight % of inorganic filler material is included in the secondary coil bobbin, the internal stress a therein can be further reduced.
Although the denaturated PPE is excellent in its adhesiveness with the epoxy resin serving as the insulation use resin, and further the moldability and insulation property thereof are desirable which contribute to stabilize the quality of the secondary coil bobbin, however, if it contains an inorganic filler material of less than 20 weight %, the linear expansion coefficient difference with other constituting members (such as the center core, the primary coil and the secondary coil) enlarges and the internal stress (thermal stress) "sgr" increases. For example, according to CAE analysis examples performed, when there is no decreases in the above mentioned "sgr", and when the temperature of the ignition coil is suddenly reduced under a temperature environment of 130xc2x0 C.xcx9cxe2x88x9240xc2x0 C., the internal stress a generated in the secondary coil bobbin showed a large value of about 90 MPaxcx9c100 MPa. Contrary thereto, according to the present invention the internal stress a can be reduced below 70 MPa, thereby, the longitudinal direction cracking in the secondary coil bobbin can be prevented. Further, as an optimum example which can reduce the internal stress "sgr" while maintaining the moldability (resin flowability), the present invention proposes a material constituted by 45 weight % xcx9c60 weight % of denaturated PPE, 15 weight % xcx9c25 weight % of glass fiber and 15 weight % xcx9c35 weight % of inorganic filler material in a non-fiber shape, the details of which will be explained in the description of the embodiments below.
Further, in view of the fact that it is preferable to vary linear expansion coefficient of a bobbin concerned for reducing the internal stress "sgr" in the bobbin, when the resin flowing direction for the resin molding is the bobbin axial direction, a desirable result was obtained when the linear expansion coefficient in orthogonal direction, (which corresponds to the radial direction and the circumferential direction of the bobbin, and an important point for preventing the longitudinal direction cracking of the bobbin is in particular, to suppress the internal stress in the circumferential direction) with respect to the resin flowing direction is 35xcx9c75xc3x9710xe2x88x926 in average at a temperature range xe2x88x9230xc2x0 C. xcx9cxe2x88x9210xc2x0 C. based on a testing method conformed to ASTM D696 in the above referred to limited containing range of the inorganic filler material, of which details will also be explained in the description of the embodiments below.